Negative Capacitance Phenomena in Dielectric/Ferroelectric Bi-layer Structures

Abstract

The capacitor is an essential component in many electronic devices, which all essentially require a large charge storage density. Conventional methods implemented to increase the charge storage density usually include increasing the capacitor area, decreasing the dielectric thickness, using higher dielectric constant materials. However, this approach is no longer compatible with the miniaturization and lower operation voltage trends found in futuristic electronic devices, and, therefore, an alternative but fundamentally disparate method is necessary. The negative capacitance (NC) effect found in ferroelectrics (FE) can be an intriguing contender to solve this problem. However, the NC effect can be expected in the FE only when they are near the FE instability. Recently, it was found that the NC effect in a FE layer in dielectric (DE)/FE stacked materials can be stabilized if the polarizations of the DE and FE layers are strongly coupled to form a single domain structure along the lateral and perpendicular directions of the films. Under this circumstance, the FE layer in the DE/FE structure can show the NC, and several studies demonstrated that the NC effect can be observed in the hetero-epitaxial DE/FE bi-layer system. Up to now, the NC effect from the DE/FE bi-layer structure has been explained based on the linear combination of the free energies with respect to polarization of DE and FE layers, which were described by the phenomenological expression of Landau-Khalatnikov (LK). However, there were conceptual difficulties in using the displacement equation within the bi-layer structure. In this work, therefore, the authors suggest an alternative approach to the possible NC mechanism in general DE/FE bi-layer structures adopting the depolarization theory. In this model, the Landau-Devonshire (LD) theory is extended to encompass the case where the high depolarization field is present due to an imperfect polarization compensation by the interposed thin DE layer between the FE and the metal electrode. When an external bias voltage is applied to the DE/FE system to polarize the FE layer, the FE bound charge of the FE layer at the interface between the FE layer and metal electrode can be fluently compensated by free carriers in the metal electrode. However, the FE bound charge at the DE/FE interface cannot be fully compensated by the presence of the DE layer between the FE layer and the opposite metal electrode. This induces depolarization field across the FE layer, and the direction of overall field across the FE layer is opposite to the applied field direction. In order to make the total applied voltage over the DE/FE layer equal to the external voltage, a voltage which is even higher than the applied voltage must be applied to the DE layer. This corresponds to the NC phenomena, leading to the capacitance boosting effect. This work also discusses the conditions that have hindered the operation of the NC phenomena in general DE/FE systems. The charge injection across the thin DE layer during voltage sweep can largely mitigate the depolarization effect, leading to decrease in the voltage and capacitance boosting. Under this circumstance, the spontaneous polarization of the FE layer can be irreversibly switched as it is the case for a single layer FE, which may correspond to the frustration of the NC phenomena in the DE/FE layer. Next, the polarization-voltage (P-V) characteristic of Al2O3/Pb(Zr,Ti)O3 (AO/PZT) structure was examined by comparing the NC model. The specific thicknesses of the amorphous AO films were varied from 2 nm to 10 nm, where the thicknesses of PZT layer were fixed as 150 nm. The thermodynamic calculation showed that the transition from the ferroelectric-like state to the paraelectric-like state with increasing AO thickness occurs at ~3 nm thickness. This paraelectric-like state should have exhibited a NC behavior without permanent polarization switching if no other adverse effects are involved. However, experiments showed typical ferroelectric-like hysteresis loops without the NC effects, which could be explained by the carrier injection through the thin AO layer and trapping of the carriers at the AO/PZT interface. Finally, therefore, short pulse measurements were adopted not to allow sufficient time for the charge injection for the direct proof of the NC effects in DE/FE structure. From these experiment set-up, it was demonstrated that capacitance (charge) boosting effects can be observed in the Al2O3/BaTiO3(AO/BTO) bi-layer capacitors as expected from the theoretical model. This results also revealed that the hysteresis phenomenon in NC devices originated from the dielectric leakage of the dielectric layer. The suppression of charge injection via the dielectric leakage, which usually takes time, inhibits complete ferroelectric polarization switching during a short pulse time. It was also demonstrated that a non-hysteretic NC effect can be achieved only within certain limited time and voltage ranges. Although the static operation is severely hindered, the NC effect in a general DE/FE system is still favorable for modern electronic devices which have an extremely high operation speed.